For use with 68k OS this is true. But PPC OSs can't use this UAE extensions.

Does that always have to be the case? Does the emulated PPC not have access to these extensions?

I can see that uae.audio source code (provided with WinUAE) is in 68k ASM .

Could an OS4.1 developer re-work the source into PPC ASM or C, add any OS4 AHI changes (presumably the existing one is for OS3.x AHI) and then re-compile/assemble a PPC UAE.audio device so it would work as a native PPC driver in WinUAE?